Hot melt adhesives, such as those used in the manufacturing of disposable diapers, packaging, and other products, are commonly formed from solid or semi-solid pieces of thermoplastic material that are melted prior to use. The thermoplastic material is available in bulk, in such solid forms as chicklets, pellets, pillows, and bricks, etc., and may be converted into a molten state in a tank having side walls and a heated bottom or base section.
The thermoplastic material is typically maintained in a molten state in the tank in sufficient volume to supply one or more applicators or dispensers. If an application requires a substantial volume of hot melt material, a substantially large volume of thermoplastic material must be maintained in a molten state, necessitating a long start up time for the apparatus. This may be especially true when starting a hot melt adhesive supply system that has been shut down overnight. Because the melted material from the prior day's use typically cools and solidifies when the system is shut down, a substantial portion of the tank may be occupied by a solid slug of thermoplastic material at start up. In addition to limiting the total output rate of the hot melt adhesive supply system, the long start up time may also result in prolonged exposure of at least some of the molten material to heat and/or to oxygen, which can cause the thermoplastic material to char, oxidize, or otherwise degrade.
As a result, some manufacturers have found it desirable to provide hot melt adhesive supply systems in which the bottom surface of the tank is sloped downwardly towards a central outlet. One or more heating elements that are cast into the base of the tank heat the bottom surface to a temperature at which the solid thermoplastic material at the bottom of the tank melts and converts to a molten state. Due to the sloped nature of the bottom surface, the newly melted material is able to flow down into the central outlet, allowing the remaining solid thermoplastic material to shift closer to the heated bottom surface. By quickly draining the newly formed liquid adhesive, such an arrangement helps reduce the total amount of thermoplastic material that is maintained in a molten state for prolonged periods of time.
One challenge associated with this type of hot melt adhesive supply system is that the funnel-shaped tank design can occasionally create a “choke point” for the melted material. For example, as the bottom of a slug of adhesive melts during start-up, the rest of the slug drops down in the middle of the tank and may impede the flow of the newly melted material to the central outlet. This may cause an undesirable reduction in material flow rate. Furthermore, current hot melt supply systems do not take maximum advantage of the thermal conductivity of the tank walls. When the cast-in heater is turned on, some of the first material to melt is located around the lower perimeter of the tank interior. The remaining solid material prevents this newly melted material from making its way towards the central outlet. If it were possible to drain this material more quickly, the overall system could be operated more efficiently and at less cost.
Therefore, a need exists for an apparatus for melting thermoplastic material that heats the material in a more productive manner and provides a less restricted flow path for the material to exit the apparatus.